A new study has revealed that ocean warming poses a significant threat to Prochlorococcus a tiny marine microbe responsible for producing nearly a third of the Earth’s oxygen. Contrary to previous assumptions that this heat-loving organism would thrive in warming oceans, the research shows that rising temperatures are causing its growth to slow, with potentially dire consequences for the planet’s food webs and atmosphere.
A Critical Microbe Under Threat
Prochlorococcus is a microscopic cyanobacterium and is considered the most abundant photosynthetic organism on Earth. It inhabits over 75% of the sunlit surface waters, particularly in the tropics, where it forms the foundational base of many marine food webs.
However research led by oceanographer François Ribalet of the University of Washington found that the microbe’s ideal temperature range is narrower than previously thought, between 19 and 28 degrees Celsius (66 to 82 Fahrenheit). The study’s most concerning finding is that in waters warmer than 30°C (86°F), the microbe’s cell division rate drastically slowed, dropping to just one-third of the rate seen at the low end of its tolerance range.
Future Projections and Global Impact
Using data from 90 research voyages spanning 13 years, the scientists projected the microbe future under different warming scenarios. The study published in Nature Microbiology, estimates that by the end of this century, Prochlorococcus productivity in tropical waters could fall by 17% under a moderate warming scenario and by 51% with more severe warming.
Globally, this could lead to a drop in the microbe’s productivity by 10% under moderate warming and 37% under the more extreme scenario.
“Their geographic range is going to expand toward the poles, to the north and south,” Ribalet explained. “They are not going to disappear, but their habitat will shift.”
The study suggests that Prochlorococcus’s simplified genome, an adaptation to nutrient-poor waters, may have led to the loss of genes that help it cope with heat stress. This could be a critical vulnerability as ocean temperatures continue to rise.
This potential decline also raises concerns about the stability of marine ecosystems. If another cyanobacteria group, like Synechococcus, were to replace Prochlorococcus, the long-established interactions within food webs could be disrupted, with unknown consequences for other organisms that have relied on it for millions of years.